1. Field of the Invention
This invention generally relates to plasmonic displays and, more particularly, to a method for improving the stability of metallic nanostructures used in the fabrication of plasmonic displays having an improved short wavelength response.
2. Description of the Related Art
Reflective display or color-tunable device technology is attractive primarily because it consumes substantially less power than liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. A typical LCD used in a laptop or cellular phone requires internal (backlight) illumination to render a color image. In most operating conditions the internal illumination that is required by these displays is in constant competition with the ambient light of the surrounding environment (e.g., sunlight or indoor overhead lighting). Thus, the available light energy provided by these surroundings is wasted, and in fact, the operation of these displays requires additional power to overcome this ambient light. In contrast, reflective display technology makes good use of the ambient light and consumes substantially less power.
One challenge for reflective displays is the achievement of high quality color. In particular, most reflective display technologies can only produce binary color (color/black) from one material set. Because of this, at least three sub-pixels using different material sets must be used when employing a side-by-side sub-pixel architecture with fixed colors. This limits the maximum reflected light for some colors to about ⅓, so that the pixels of this type cannot produce saturated colors with a good contrast.
In polymer-networked liquid crystal (PNLC) or polymer dispersed liquid crystal (PDLC) devices, liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the “smart window”. Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that includes a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor.
Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This scattering results in a translucent “milky white” appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals align completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used. It is also possible to create fire-rated and anti X-Ray versions for use in special applications. Most of the devices offered today operate in on or off states only, even though the technology to provide for variable levels of transparency is available. This technology has been used in interior and exterior settings for privacy control (for example conference rooms, intensive-care areas, bathroom/shower doors) and as a temporary projection screen.
The full range of colors produced by plasmon resonances resulting from metal nanostructures has been known since ancient times as a means of producing stained colored glass. For instance, the addition of gold nanoparticles to otherwise transparent glass produces a deep red color. The creation of a particular color is possible because the plasmon resonant frequency is generally dependent upon the size, shape, material composition of the metal nanostructure, as well as the dielectric properties of the surrounding environment. Thus, the optical absorption and scattering spectra (and therefore the color) of a metal nanostructure can be varied by altering any one or more of these characteristics. The parent applications listed above describe means of electronically controlling these color-producing characteristics.
The properties of metallic nanoparticles have drawn significant attention due to their application in photonics and electro-optics, as well as their potential application in biological/chemical sensors and renewable energy. Moreover, the fabrication of periodic metal nanoparticle arrays for applications in photonics utilizing their localized surface plasmon resonance (LSPR) properties has been extensively studied in recent years. Among various processing techniques, depositing a film of metal on a nano-size patterned mask and using a lift-off process to remove the sacrificial layer is becoming a widely used technique, because it allows for fabricating nanoparticles with precisely controlled shape, size, and particle spacing.
Ag metallic nanoparticles are generally used as the material of choice for plasmonics due to its strong plasmonic response across the visible and near infrared (IR) wavelength range, while other materials support a strong resonance only in a narrow wavelength range. Specifically, many of the characteristics depend on the permittivity value at specific wavelengths. The real part of the permittivity indicates how the electrons are driven by the electromagnetic field, and a large negative real part of permittivity is desirable for plasmonic devices due to the strong polarization induced by the external electric field of incident light. As the wavelength decreases, the real permittivity decreases in magnitude for all the metals except Al in which the value stays large, even in the blue wavelength range. The imaginary part of the permittivity represents the losses encountered in polarizing the material. Although Al is lossy at the red part of the spectrum, it has lower loss than materials such as Au at short wavelengths. This combination of a large negative real part and a small imaginary part of permittivity indicates that Al is a viable metal for plasmonics, especially at short visible wavelengths.
However, a problem arises when plasmonic nanoparticles are made of Al because the material undergoes oxidation in ambient atmosphere, to form aluminum oxide layer shell. The oxide formed at the surface of Al nanoparticles generates plasmon resonance shifts due to refractive index change at the metal-dielectric interface. Moreover, the approach to tuning the plasmon resonances by changing the dielectric constant of the surrounding environment of the nanoparticles is significantly affected by the dielectric oxide shell formed at the surface of Al nanoparticles.
FIGS. 1A and 1B show an example of a metal nanoparticle configured to reflect incident light at visible wavelengths due to localized surface plasmon. (LSPR) enhancement (prior art). With an initial surrounding medium having a refractive index of 1.5, reflected light is in the green range of the visible spectrum (FIG. 1A). When the refractive index of the medium is switched from 1.5 to 2.0, the reflected light is red-shifted to a longer wavelength of the visible spectrum (FIG. 1B).
FIGS. 2A and 2B, in contrast to FIGS. 1A and 1B, show an example of a metal nanoparticle surrounded by an oxide shell (prior art). If the oxide shell has a similar refractive index compared to its surrounding medium, the resulting light that is reflected in FIG. 2A is similar to FIG. 1A. However, tuning the refractive index of the surrounding medium to 2.0 (FIG. 2B) does not result in a significant change in the plasmon resonance because the near-field enhancement of localized plasmon-polaritons is strongest and most efficient at the immediate interface of metal-dielectric layers.
It would be advantageous if new structures existed to achieve the tuning of plasmonic resonances across a wide range of visible spectrum, especially in the blue regime, as required for display-related and optical filter applications.